Silicon nitride-alumina composite ceramics and producing method thereof

ABSTRACT

Silicon nitride-alumina composite ceramics composed of silicon nitride crystals, α-alumina crystals and β&#39;-sialon crystals, wherein the silicon nitride crystals and the β&#39;-sialon surrounded the α-alumina crystals so as to prevent the connection of the α-alumina crystals. And a method for producing the silicon nitride-alumina composite ceramics which has the steps of preparing a mixture powder composed of 20 to 70 wt % of alumina powder and silicon nitride powder as a remainder, the average particle diameter of the alumina powder being two or more times as large as that of the silicon nitride powder, and firing the mixture powder in an atmosphere of inactive gas.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to silicon nitride-alumina compositeceramics suitable to the material for a heater supporting member of aceramic glow plug, a base plate of an IC pressure sensor and the like.

2. Description of the Prior Art

β'-sialon[Si_(6-z) Al_(z) O_(z) N_(8-z) (Z=0 to 4.2)] is well known asthe silicon nitride-alumina ceramics.

β'-sialon is a complete solid solution of silicon nitride and α-alumina,of which the coefficient of thermal expansion is as small as 3.0×10⁻⁶/°C. in the temperature range from room temperature to 1000° C., similarto that of silicon nitride.

The material having a small coefficient of thermal expansion generallyexhibits excellent thermal shock resistance, but the use thereof islimited since damage often occurs due to the difference in coefficientof thermal expansion when such material is jointed to another materialsuch as metal.

By preparing a mixture powder of silicon nitride and alumina in anadjusted ratio and sintering the mixture powder under adjustedconditions, not complete solid solution but composite ceramicscontaining unreacted silicon nitride and α-alumina can be obtained. Byadjusting the amount of the unreacted alumina, the coefficient ofthermal expansion of the obtained ceramics can be adjusted.

However, the experimental results on many samples of ceramics containingunreacted α-alumina show that the strength thereof is scattered and someone exhibits rather low strength. This scattering of strength is notobserved in the material containing only one of silicon nitride andβ'-sialon.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide ceramics composed ofsilicon nitride which imparts high strength to ceramics, α-alumina whichadjusts the coefficient of thermal expansion of ceramics and β'-sialonwhich strongly bonds silicon nitride to α-alumina, and exhibitingscattering properties, and the producing method thereof.

After many studies and experiments, the present inventors have foundthat the scattering in strength of silicon nitride-alumina compositeceramics results from the crystal structure of α-alumina.

Namely, when the silicon nitride(Si₃ N₄) powder is mixed withalumina(Al₂ O₃) powder as shown in FIG. 2 and is sintered, reaction offorming β'-sialon in the interfaces between silica nitride particles andalumina particles is produced. However, the interfaces between siliconnitride particles and unreacted alumina particles are not completelyburied with β'-sialon but alumina particles (of which the strength islower than that of silicon nitride) are connected to one another withoutintervention of silicon nitride particles. This results in cracks beingproduced in these connected portions of alumina particles andaccordingly, the decrease in strength of obtained ceramics beingobserved.

According to the present invention, silicon nitride-alumina compositeceramics has a crystal phase mainly composed of silicon nitride,α-alumina and β'-sialon produced by the reaction of silicon nitride andα-alumina, wherein α-alumina crystals are surrounded by silicon nitrideand β'-sialon crystals so as not to be connected one another.

In this case, silicon-alumina composite ceramics of which the totalamount of silicon nitride and β'-sialon crystals is 45 to 90 wt % andthe amount of alumina crystals is 10 to 55 wt %, exhibits high strengthsubstantially equal to that of silicon nitride. And in accordance withthe amount of alumina crystals, the coefficient of thermal expansion canbe varied in the range from about 3.2 to 6.1×10⁻⁶ /°C.

The ceramics having the above properties, is obtained by preparing amixture power composed of 20 to 70 wt % of alumina powder and siliconnitride powder as the remainder, the average particle diameter ofalumina powder being about two or more times as large as that of siliconnitride powder (FIG. 1) and firing the prepared mixture powder in anatmosphere of inactive gas.

When the amount of the alumina powder in the mixture powder is smallerthan 20 wt %, almost all alumina powder reacts on silicon nitride powderto be changed into β'-sialon. This respectively alumina crystalsremaining, and accordingly, the adjustment of coefficient of thermalexpansion becoming impossible.

When the amount of alumina powder is in the range of 20 wt % to 70 wt %,the alumina powder having the average particle diameter of about two ormore times as large as that of silicon nitride must be used in order tosurround α-alumina crystals by silicon nitride and β'-sialon crystals.

When the amount of alumina powder is larger than 70 wt %, even if theparticle diameter of alumina powder and silicon nitride powder isadjusted, α-alumina crystals cannot be surrounded by silicon nitride andβ'-sialon crystals.

The ceramics according to the present invention, the alumina crystals ofwhich the strength is low, are not connected to one another, butsurrounded by silicon nitride and β'-sialon crystals. Therefore, in theceramics according to the present invention, cracks are scarcelyobserved as compared with the conventional ceramics of this type. Themixture powder used in the method according to the present invention,has the state that silicon nitride particles of small particle diameterintervene between alumina particles of larger particle diameter so as tosurround alumina particles as shown in FIG. 1.

By firing this mixture powder, α-alumina crystals become surrounded bysilicon nitride and β'-sialon crystals.

It was confirmed by experiments that the strength of ceramics isscarcely decreased in spite of the increase in amount of aluminacrystals in the range of 10 to 55 wt %.

BRIEF EXPLANATION OF THE DRAWINGS

FIG. 1 is a diagram showing the mixing state of a low material ofsilicon nitride-alumina composite ceramics according to the presentinvention.

FIG. 2 is a diagram showing the mixing state of the conventional lawmaterial;

FIGS. 3, 4, 5, and 6 show the experimental result on the presentinvention, respectively;

FIG. 6 is a sectional view of a glow plug provided with a ceramic heaterwherein silicon nitride-alumina composite ceramics according to thepresent invention is applied to a heater supporting member; and

FIG. 8 is a view showing the producing method of the ceramic heater.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

Hereinafter, several experimental results on the strength of ceramicswill be explained.

EXPERIMENT 1

Hereafter, silicon nitride powder denotes a mixture powder of 90.0 wt %of silicon nitride(Si₃ N₄)powder, 5.0 wt % of spinel(MgAl₂ O₄)powder and5.0 wt % of yttria(Y₂ O₃) powder. The spinel powder has an averageparticle diameter of 1.6 μm, the yttria powder has an average particlediameter of 1.2 μm. The spinel and yttria act as sintering auxiliaryagent. One of spinel and yttria can be also used.

The average particle diameter of silicon nitride powder denotes that ofsilicon nitride within the above described mixture powder.

The strength denotes the simply averaged value of the three pointbending deflection strength of 20 samples at a normal temperature. Eachsample has a polished surface and has a plate shape of 3.0×4.0×40.0 mm.In this experiment, the crosshead speed is 0.5 mm/min, the span is 30.00mm.

The coefficient of thermal expansion denotes the average value of threesamples measured in the temperature range of room temperature to 1000°C. In this case, the increasing rate of temperature is 5° C./min.

Silicon nitride powder and α-alumina (hereafter will be called onlyalumina) powder, each having variously different average particlediameters are mixed with each other together with an organic solvent,and the mixture powder is formed by the doctor blade method to obtain aplurality of ceramic sheets. The obtained ceramic sheets are piled onone another and laminated at about 120° C. Then, the laminated body isheld in an atmosphere of inactive gas such as argon(Ar) annitrogen(N₂)gas at 1600° C. for 30 to 60 minutes, and hot-pressed undera pressure of 500 kgf./cm² to obtain a silicon nitride-alumina compositeceramics.

The properties of the obtained ceramics are examined.

Table 1 shows the examined properties. FIG. 3 shows the relation betweenthe composition ratio of silicon nitride powder and alumina powder andthe deflection strength based on the result of Table 1, and FIG. 4 showsthe relation between the above composition ratio and the coefficient ofthermal expansion.

In Table 1, the particle diameter ratio denotes the following ratio:##EQU1##

Table 1 and FIG. 4 clearly show that the coefficient of thermalexpansion is substantially equal in the same composition range ofsilicon nitride and alumina regardless of the particle diameter of thelaw material.

In contrast, the deflection strength is considerably influenced by theparticle diameter of the law material as is apparent from Table 1 andFIG. 3.

For example, when the average particle diameter of silicon nitrideparticles is about ten times as large as that of the alumina particles,namely when the particle diameter ratio is 0.10, the strength of theobtained ceramics is decreased to that substantially equal to that ofalumina in the case that the composition ratio of the alumina particlesis about 20 wt % or more. In contrast, when the average particlediameter of alumina particles is about 10 times as large as that ofsilicon nitride particles, namely when the particle diameter ratio is10.38, the strength of the obtained ceramics is nearly equal to that ofsilicon nitride even in the case that the composition ratio of aluminaparticles is 70 wt %. However, the relation between the strength and theparticle diameter of the law material is not a linear one. When theaverage particle diameter of silicon nitride particles is nearly equalto that of alumina particles, namely, when the particle diameter ratiois 1.04, the obtained strength is not a middle value thereof but isshifted to the side of alumina particles of lower strength. Namely, inthe mixing state shown in FIG. 2, the silicon nitride particles do notreact on the alumina particles. The alumina particles having lowstrength are partially connected. This results in the average strengthof the obtained ceramic being shifted to the side of alumina particlesas shown in FIG. 3.

In contrast, when the particle size of the silicon nitride particleshaving high strength is made smaller than that of the alumina particleshaving low strength, the alumina particles are prevented from beingconnected to one another in the mixing state shown in FIG. 1. Thisresults in high strength being obtained as shown in FIG. 3.

When the composition ratio of the alumina particles is small, thedecrease in strength is not observed and the coefficient of thermalexpansion is a small value nearly equal ot that of silicon nitride. Thisresults from the alumina particles reacting on the silicon nitrideparticles to be changed into β'-sialon and accordingly no unreactedalumina particles remaining.

As is known from Table 1 and FIG. 3, the ceramics composed of 80 to 30wt % of silicon nitride and 70 to 20 wt % of alumina of which theparticle diameter is increased to have a particle diameter ratio of10.38, always exhibits high strength over the wide composition range ofalumina.

It was confirmed that the ceramics having the particle diameter ratio of10.38 is composed of silicon nitride crystals, α-alumina crystals andβ'-sialon crystals formed by the reaction therebetween, and thatα-alumina crystals are disconnected and surrounded by the siliconnitride and β'-sialon crystals.

Furthermore, it was also confirmed that the coefficient of thermalexpansion of this ceramics can be varied in the range of 3.2 to 6.1×10⁻⁶/°C. by setting the amount of silicon nitride crystals and β'-sialoncrystals to 45 to 90 wt % and setting the amount of α-alumina crystalsto 10 to 55 wt %.

EXPERIMENT 2

50 wt % of silicon nitride powder and 50 wt % of alumina powder, ofwhich the average particle diameter is variously changed from eachother, are mixed and formed into silicon nitride-alumina compositeceramics having variously different particle diameter ratios in similarmethod to Experiment 1. And the properties of the obtained ceramics areexamined. The result of the examination is shown in Table 2 and FIG. 5.

As is apparent from FIG. 5, the strength of the ceramics is increased asthe increase in particle diameter ratio. In FIG. 5, an average strengthof 100 samples, which is obtained by using Weibull probability paper, isshown. When the particle diameter ratio is logarithmically indicated,the average strength is almost linearly increased in proportion to theparticle diameter ratio.

The maximum strength and the minimum strength are changed along an Scurve, respectively, each having an inflection point at the particlediameter ratio of about 1.0. Especially, the minimum strength isinreased over the particle diameter ratio of about 1.0 and remarkablyincreased over the particle diameter ratio of 2.0.

From FIG. 5, it can be concluded that the preferable particle diameterratio is 2.0 or more and more preferable ratio is 4.0 or more. SamplesNos. 6 to 9 in Table 2 have the above described preferable particlediameter ratio.

The coefficient of thermal expansion of three samples randomly selectedfrom each of samples Nos. 1 to 9 (total number of the measured samplesis 27) which is measured in the temperature range of room temperature to1000° C., is within the range of 4.75±0.1×10⁻⁶ /°C.

EXPERIMENT 3

50.0 wt % of silicon nitride powder and 50.0% of alumina powder aremixed by changing the average particle diameter of silicon nitridepowder and alumina powder so as to have a particle diameter ratio of 4.2to 4.6. The obtained mixture powder is sintered by the method similar tothat of Experiment 1. And the properties of the obtained siliconnitride-alumina composite ceramics are examined. The result of theexperiment was shown in Table 3 and FIG. 6.

As is apparent from FIG. 6, when the average particle diameter of thesilicon nitride particles and alumina particles of the ceramics havingthe same particle diameter ratio, are increased, the strength thereof isdecreased and scattered in an enlarged range.

This seems to result from the preferable mixing state as shown in FIG. 1being not obtained when the particle diameter of silicon nitrideparticles and alumina particles is excessively increased.

Namely, it is considered that many portions of the surface of aluminaparticles do not come in contact with the silicon nitride particles andalumina particles are not surrounded by β'-sialon phase, andaccordingly, cracks are generated in the alumina particles. Under theabove circumstances, it is preferable to use alumina particles having anaverage particle size of about 10μm or less.

In addition, other experimental results on the particle diameter showthat the preferable average particle size of silicon nitride is about2μm or less.

The average strength is obtained by simply averaging the strength of tensamples.

EXPERIMENT 4

Hereinafter, the results of endurance tests on the siliconnitride-alumina composite ceramics applied to a glow plug for a dieselengine will be explained.

In the glow plug shown in FIG. 7, a heater element 1 having a letter Ushaped section is jointed to a tip end of a rod-shaped heater supportingmember 2. A pair of lead wires 3a and 3b made of tungsten are embeddedin the supporting member 2 in its axial direction. An end of each of thelead wires 3a and 3b is connected to the heater element 1. A metallicsleeve 4 is installed around the supporting member 2 and a metallic body5 is installed around the metallic sleeve 4.

Another end of the lead wire 3a extends to a base end of the supportingmember 2 and is connected to a metallic cap 6 fit on the base end of thesupporting member 2 to be electrically connected to an electric powersource (not shown) through a cap 6 and a nickel wire 7.

Another end of the lead wire 3b is electrically connected to themetallic body 5 through the metallic sleeve 4.

The glow plug having the above described structure is secured in a walldefining a combustion chamber of an engine(not shown) by a screw 51formed in the metallic body 5 so as to penetrate the wall.

The heater element 1 is composed of a sintered body made of a mixturepowder of conductive molybdenum disilicide (MoSi₂) and insulatingsilicon nitride. MoSi₂ imparts oxidization resistance to the heaterelement 1 and Si₃ N₄ imparts thermal shock resistance thereto.

The supporting member 2 is composed of silicon nitride-alumina compositeceramics made of a sintered body of a mixture powder of silicon nitrideand alumina. The supporting member 2 is integrally sintered with theheater element 1.

FIG. 8 shows the producing method of a ceramic heater.

Ceramic sheets 1' for forming the heater element 1 and ceramic sheets 2'for forming the supporting member 2 are assembled and piled on oneanother as shown in FIG. 8. And the lead wires 3a and 3b are sandwichedbetween the ceramic sheets 1' and 2' and then the obtained body ishot-pressed at 1600° C. under 500 kg/cm² thereby to obtain a ceramicheater.

In operation, an electric current flows to the heater element 1 throughthe nickel wire 7, the metallic cap 6 and the lead wire 3a to generateheat, and is grounded through the lead wire 3b, the metallic sleeve 4and the metallic body 5.

Next, ceramic heaters, each of which has the supporting member 2 made ofsilicon nitride-alumina composite ceramics formed by methods equal tothose of Samples Nos. 3, 5 and 7 of Experiment 2, are prepared and areinstalled in glow plugs. Then, endurance tests are conducted on theseglow plugs. In this case, each heater element is made of 71.7 wt % ofmolybdenum disilicate and 28.3 wt % of silicon nitride. And theresistance of this heater element is 0.18Ω at a normal temperature.

The endurance tests are conducted by the following method: Voltage isset so that the equilibrium temperature is 1300° C. This set voltage isintermittently applied to each glow plug in the cycle of one minutevoltage application and one minute voltage nonapplication. And thenumber of cycles when cracks are observed in each supporting member 2,is examined.

In the glow plugs, each having the supporting member made of Sample 3 inExperiment 2, all of four supporting members being to be cracked in thevicinity of the heater element in under 3000 cycles.

In the glow plugs, each having the supporting member made of Sample 5,three out of four supporting members are similarly cracked in 3000 to4000 cycles and the remaining one supporting member is similarly crackedin 6000 to 7000 cycles.

However, in the glow plugs, each having the supporting member made ofSample 7, all of four supporting members are not cracked even in 10,000cycles.

Considering the above test results, the supporting member made of asintering body of a mixture of silicon nitride and alumina has beengiven attention in order to properly adjust the coefficient of thermalexpansion. However, silicon nitride and alumina react to form β'-sialon.This β'-sialon exhibits high strength similar to that of silicon nitridebut the coefficient of thermal expansion thereof is low similar to thatof silicon nitride.

Under the above circumstances, the composition ratio of alumina withinthe mixture powder has been gradually increased in order to make a partof alumina crystals within the sintering body unreacted and adjusts thecoefficient of thermal expansion by the unreacted alumina crystals. Thisresults in the strength of the ceramics being widely scattered andaccordingly, there sometimes occurs such a problem as the production ofsupporting members of low strength.

According to the present invention, the above described heater elementcan be supported by the heater supporting member having a coefficient ofthermal expansion similar to that of the heater element, heat resistanceand unscattered strength. In the supporting member for a heater elementfor a glow plug, it is preferable to use such alumina powder as havingan average particle diameter of about four times as large as that ofsilicon nitride.

The heater element for a ceramic glow plug is desired to have excellentoxidization resistance, small specific resistance and a largecoefficient of thermal expansion. A sintered body of a mixture ofmolybdenum disilicide and silicon nitride is well known as the materialof the heater element having the above described properties.

The most preferably composition ratio of the above mixture is 71.1 wt %of molybdenum disilicide and 28.3 wt % of silicon nitride as describedabove. The coefficient of thermal expansion of the heater element havingthe above composition ratio is about 4.4 to 5.2×10⁻⁶ /°C. in thetemperature range of room temperature to 1000° C.

The supporting member formed according to the present invention, whichsupports the heater element of the ceramic glow plug must haveinsulating properties a, coefficient of thermal expansion similar tothat of the heater element, heat resistance and strength durability foruse within an engine.

The supporting member made of silicon nitride has properties similar tothe above described conditions, but the coefficient of thermal expansionof silicon nitride is as small as about 3.0×10⁻⁶ /°C. in the temperaturerange of room temperature to 1000° C. This results in the supportingmember made of silicon nitride being damaged in joint portions with theheater element due to the difference in coefficient of thermalexpansion.

As described above, according to the present invention, siliconnitride-alumina composite ceramics having excellent strength,unscattered properties and variously different coefficients of thermalexpansion can be obtained.

Therefore, the ceramics according to the present invention can besuitably used in a heater supporting member of a ceramic glow plug, abase plate of an IC pressure sensor and the like.

                                      TABLE 1                                     __________________________________________________________________________            Average Particle Diameter [μm]                                             Si.sub.3 N.sub.4                                                                    Al.sub.2 O.sub.3                                                                     Si.sub.3 N.sub.4                                                                    Al.sub.2 O.sub.3                                                                     Si.sub.3 N.sub.4                                                                    Al.sub.2 O.sub.3                              2.22  0.23   0.49  0.51   0.21  2.18                                          Particle Diameter Ratio [-]                                                   0.10         1.04         10.38                                               Composition Ratio [wt %]                                                            Coefficient  Coefficient  Coefficient                                         of Thermal   of Thermal   of Thermal                                    Strength                                                                            Expansion                                                                            Strength                                                                            Expansion                                                                            Strength                                                                            Expansion                             Si.sub.3 N.sub.4                                                                  Al.sub.2 O.sub.3                                                                  [kgf./mm.sup.2 ]                                                                    [× 10.sup.-6 /°C.]                                                      [kgf./mm.sup.2 ]                                                                    [× 10.sup.-6 /°C.]                                                      [kgf./mm.sup.2 ]                                                                    [× 10.sup.-6 /°C.]       __________________________________________________________________________    100  0  87.4  3.20   88.6  3.18   90.1  3.18                                  90  10  76.1  3.20   83.8  3.19   88.8  3.21                                  80  20  50.6  3.21   74.8  3.24   86.7  3.28                                  70  30  43.3  3.58   62.4  3.66   85.2  3.65                                  60  40  40.5  4.18   53.0  4.22   84.8  4.24                                  50  50  39.2  4.65   48.8  4.76   83.9  4.78                                  40  60  38.5  5.38   45.6  5.47   80.4  5.52                                  30  70  37.8  6.12   42.2  6.08   77.5  6.11                                  20  80  36.6  6.78   39.9  6.67   55.4  6.72                                  10  90  36.2  7.30   37.7  7.28   40.6  7.22                                   0  100 35.7  7.86   36.4  7.80   35.4  8.06                                  __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________                Sample No.                                                                    1  2  3  4  5  6  7  8   9                                        __________________________________________________________________________    Si.sub.3 N.sub.4 Average                                                                  4.26                                                                             2.22                                                                             1.02                                                                             1.02                                                                             0.49                                                                             0.49                                                                             0.21                                                                             0.21                                                                              0.21                                     Particle Diameter[μm]                                                      Al.sub.2 O.sub.3 Average                                                                  0.23                                                                             0.23                                                                             0.23                                                                             0.51                                                                             0.51                                                                             1.07                                                                             1.07                                                                             2.18                                                                              4.33                                     Particle Diameter[μm]                                                      Particle Diameter                                                                         0.05                                                                             0.10                                                                             0.22                                                                             0.50                                                                             1.04                                                                             2.18                                                                             5.10                                                                             10.38                                                                             20.62                                    Ratio(Al.sub.2 O.sub.3 /Si.sub.3 N.sub.4)[-]                                  50% Average Strength                                                                      31.2                                                                             34.6                                                                             39.5                                                                             44.0                                                                             53.0                                                                             63.7                                                                             76.1                                                                             82.6                                                                              87.0                                     [kgf./mm.sup.2 ]                                                              Maximum Strength                                                                          41.8                                                                             48.3                                                                             56.9                                                                             77.2                                                                             92.6                                                                             96.5                                                                             98.2                                                                             100.6                                                                             10.25                                    [kgf./mm.sup.2 ]                                                              Minimum Strength                                                                          16.5                                                                             17.0                                                                             19.3                                                                             21.0                                                                             25.4                                                                             40.2                                                                             60.3                                                                             68.8                                                                              74.7                                     [kgf./mm.sup.2 ]                                                              Weibull Coefficient                                                                       16.2                                                                             14.1                                                                             9.8                                                                              5.9                                                                              4.1                                                                              6.4                                                                              10.1                                                                             13.9                                                                              15.7                                     __________________________________________________________________________

                  TABLE 3                                                         ______________________________________                                        Sample No.   1       2       3     4     5                                    ______________________________________                                        Si.sub.3 N.sub.4 Average                                                                   0.49    1.02    2.22  4.26  8.05                                 Particle                                                                      Diameter[μm]                                                               Al.sub.2 O.sub.3 Average                                                                   2.18    4.33    10.06 18.32 35.74                                Particle                                                                      Diameter[μm]                                                               Particle     4.45    4.25    4.53  4.30  4.44                                 Diameter                                                                      Ratio                                                                         (Al.sub.2 O.sub.3 /Si.sub.3 N.sub.4)[-]                                       Average Strength                                                                           75.4    70.8    71.2  55.3  46.7                                 [kgf./mm.sup.2 ]                                                              Maximum Strength                                                                           83.3    77.0    84.1  65.9  59.5                                 [kgf./mm.sup.2 ]                                                              Minimum Strength                                                                           68.2    65.1    59.2  44.2  30.8                                 [kgf./mm.sup.2 ]                                                              ______________________________________                                    

What is claimed is:
 1. Silicon nitride-alumina composite ceramics composed of a sintered body of a mixture of alumina particles and silicon nitride particles, the average particle diameter of said alumina particles being two or more times as large as that of said silicon nitride particles, said ceramics including silicon nitride crystals, α-alumina crystals and β'-sialon crystals, the total amount of said silicon nitride crystals and said β'-sialon crystals being 45 to 90 wt %, and the amount of said α-alumina crystals being 10 to 55 wt %, said silicon nitride crystals and said 'sialon crystals surrounding said α-alumina crystals so as to prevent the connection of said α-alumina crystals.
 2. A heater supporting member for a ceramic glow plug, composed of a sintered body of a mixture of alumina particles and silicon nitride particles, the average particle diameter of said alumina particles being two or more times as large as that of said silicon nitride particles, and said sintered body having a structure wherein silicon nitride crystals and β'-sialon crystals surround α-alumina crystals so as to prevent the connection of said α-alumina crystals.
 3. Silicon nitride-alumina composite ceramics according to claim 1, wherein said mixture being composed of 20 to 70 wt % of alumina particles and silicon nitride particles as a remainder and the average particle diameter of said silicon nitride particles is 2μm or less.
 4. A heater supporting member according to claim 2, wherein said mixture being composed of 20 to 70 wt % of alumina particles and silicon nitride particles as a remainder and the average particle diameter of said silicon nitride particles is 2μm or less. 